In cellulose manufacturing processes in which wood chips are heated in a cooking solution under pressure, about half of the wood is dissolved, forming the so-called waste liquor. One of the main organic components of waste liquor is lignin. In the sulphite process, in which an acid bisulphite/sulphur dioxide solution serves as digesting liquor, lignin is present in the waste liquor in sulphonated form, as so-called lignosulphonates. In this form, lignin is soluble in water also in acid solutions. On the other hand, in alkaline cellulose manufacturing processes, such as the soda process (NaOH as digesting solution), in the kraft process (NaOH, Na.sub.2 S, NaHS as digesting solution) or in the oxygen/alkali process (with NaOH in the presence of oxygen), lignin is present in the form of an Na salt in the waste liquor, and it is water-soluble in alkaline solutions.
It is known in the art that the characteristics of e.g. paper, cardboard and fibre boards can be improved by adding lignin to the fibres, most often in the form of waste liquor. The problem has, however, been the poor retention of said lignin products, that is, poor adhesion to the fibres. For instance, it has been possible to improve the strength properties of corrugated board by adding to the fibre stock 6% by weight, calculated on the dry matter of the stock, thermally processed lignin isolated from sulphite waste liquor (Zellstoff und Papier 24 (1975): 9, 269-70). It is observed, however, in the reference that using lignin cannot be contemplated because the BOD values of the waste liquor become too high owing to its poor retention.
In the Canadian Patent CA 729,140 lignin is used in the form of sulphite waste liquor or black liquor for improving the strength characteristics of paper. Of the dry matter of the black liquor only 26% by weight could be made to adhere to the fibres. All the same, the procedure was considered to be economically advantageous because the concora values of the paper increased from 66.3 points to 87.6 points. A retention of equal degree, about 30% by weight, has also been reached in a Russian study (Bumazh Prom. 11 (1984): 18-19), in which kraft lignin was precipitated with alum onto the paper fibres. The lignin quantity used was over 8% by weight of lignin dry matter, calculated on the dry matter of the fibres. In spite of poor retention, the breaking, punching and rigidity characteristics of the paper could be improved.
The effect of kraft lignin on the strength of fibre board made from waste paper has been compared in an American study (Forest Products J. 28 (1978): 77-82) with the effect of phenol formaldehyde resin. Of the dry matter of the kraft lignin product used, 54.8% by weight was organic matter and 44% by weight was inorganic matter. When pH was adjusted with sulphuric acid to 4.5, only 15 g dry matter out of 100 g were precipitated. Precipitation could be somewhat improved by raising the precipitation temperature to 70.degree. C.; although lignin had to be added 11% by weight, calculated as dry matter related to the dry matter of the fibres, in order to achieve equally good strength characteristics as with 3% by weight phenol formaldehyde addition. Cost calculation analysis revealed that phenolic resin was somewhat more favourable in use than lignin owing to higher lignin addition and oxygen consumption, and to the heating required in precipitating it.
Corrugated board manufactured from reclaimed fibre stock has been impregnated, in Italy, in a sizing press with both NSSC waste liquor and black liquor (Eucepa Conference, Florence, Oct. 6-10, 1986, Proc. Vol. II, 24; 1-31). Subsequent to impregnation, the board was dried at 100.degree.-140.degree. C. The waste liquors originated from poplar and straw cookings. In most of the experiments, the characteristics of the board improved owing to the impregnation. Better results were obtained using the waste liquor at dry matter constant 20-30% by weight than when it was used in dilute solutions, which necessitated performing the impregnation in a plurality of steps. On the other hand, lowering the pH of the waste liquors reduced the strength properties of the board. The board impregnated with waste liquor obtained from straw cooking absorbed more moisture than untreated corrugated board.
Lignin has been chemically modified, particularly for improving its water resistance properties. It has for instance been reacted with amines (Wochenblat fur Papierfabrikation 94 (1966): 4, 107-110), whereafter it has been reacted with formaldehyde (U.S. Pat. No. 3,079,353) or peroxide (SU 520,260). Alakli lignin has, for instance, been sulphonated and used together with starch in order to increase the rigidity of a liner manufactured from waste paper (U.S. Pat. No. 3,644,167). From rice soda black liquor, cyanoethyllignin and aminolignin have been manufactured, and they have been used e.g. in paper sizing (Paperi ja Puu 62 (1980): 10, 589-592, 614). Lignin has also been heat-treated: e.g. lignosulphonates at about 250.degree. to 300.degree. C., with the purpose of converting them into a form insoluble in water (U.S. Pat. No. 2,934,531 and Zellstoff und Papier 18 (1969): 11, 328-332). Using this kind of products it has been possible to improve the characteristics of corrugated board and kraft.
Kraft lignin, having average molar mass 25,000-30,000, has been used in manufacturing cardboard, corrugated board and wrapping paper (SU 681,140). As disclosed in said reference, the lignin is added to an aqueous solution containing 2-40% by weight fatty acids and resin acids, and the mixture is heated in an alkali at 80.degree. to 85.degree. C. before use. The dry matter content of the mixture is about 14-18% by weight and the pH is 8-9. The fatty acids and resin acids have probably been added in order to improve the hydrophobic properties. When this mixture was added to a cardboard fibre stock in amount 3% by weight, calculated as dry matter related to the dry matter of the stock, the bursting strength of the cardboard was improved 5% and the tensile strength, 18%.
High-molar mass alkaline kraft lignin has been added 1 to 7% by weight, calculated as dry matter related to the stock dry matter, to a fibre stock used in manufacturing paper and cardboard (SU 374,407). The lignin is added as an alkaline aqueous solution to the stock, and thereafter, 3% by weight of divinylstyrene/rubber latex in emulsion form, containing 75% by weight styrene related to the latex dry matter, is added thereto. Thereafter, the components are precipitated with aluminum sulphate. The rigidity of paper and cardboard could hereby be improved, in comparison with products reinforced with latex alone. The additions are comparatively low, only about 1-7% by weight lignin.
Improved absorptive paper suitable for lamination can be manufactured by adding to the paper in the sizing press, after web formation, high-molar mass lignin, of which at least 50% by weight is over 5000 in molar mass (FI 58,961). The lignin product may consist of lignosulphonates or kraft lignin, and the additions are 0.1 to 10% by weight of the dry paper weight. According to the reference, lignin accelerates the absorption of phenolic resin in the paper but does not affect the tensile strength of the paper.
In the Finnish Patent FI 58788, lignin derivatives are used, of which at least 35% by weight, advantageously 40% by weight, are over 5000 in molar mass. By mixing them with phenol formaldehyde resin and adjusting pH of the bonding agent to be in the range 8-14, preferably 9-13, a waster resistant bonding agent can be manufactured for chipboard, plywood and fibre boards.
As will be evident from the foregoing, the greatest problems in using lignin as a reinforcing agent for various materials has been caused by the poor retention of lignin products, that is, their poor adhesion to fibres.
In the Patent SU 681,140, a lignin product having a given average molar mass (25,000-30,000) has been used. Since lignin is not homogeneous in structure but consists of lignin derivatives differing greatly in molar mass, the value stated for the average molar mass in fact fails to specify the kind of the lignin product used. It may consist of very high-molar mass and very low-molar mass lignin derivatives in varying proportions so that the mean of their molar masses will be 25,000-30,000. It is probably because of the poor adhesion to fibres of low-molar mass lignin derivatives that fatty and resin acids had to be used in addition in this instance in order to improve the hydrophobic properties.
Endeavours have also been made to improve the retention of lignin and the strength and water resistance properties of fibre material chemically by modifying lignin, but the economy of the procedure has suffered in that case.